1. Field of the Invention
The present invention relates to an image sensing system including a light source, and more specifically, to an image sensing system such as an image scanner including a light source.
2. Description of the Prior Art
An image sensing system using a light source (chiefly, a fluorescent lamp) of a type which performs AC lighting and a solid-state image sensing device which accumulates charges for a predetermined period of time to output signals in time sequence presents a problem in that the output signal is unstable. This is because the charge accumulation amount of the solid-state image sensing device is not constant due to the non-uniformity of the lighting by the light source.
In an image sensing system such as a scanner including a light source shown in U.S. Pat. No. 4,691,365, since the selection of a light source can be made, even when a fluorescent lamp is used, this problem can be solved by increasing the lighting frequency (i.e. by using an inverter type fluorescent lamp).
Referring now to a block diagram of FIG. 1, a conventional image sensing system including a light source will be described. This conventional system is a film scanner including a fluorescent lamp 1, an inverter power source 2, a film holder 3, a lens 5, a CCD (change coupled device) image sensing device 6, a timing generator 7, a microcomputer 9 and a processing circuit 12.
The fluorescent lamp 1 is driven by the inverter power source 2 and irradiates light to a film 4 attached to the film holder 3. Also, the fluorescent lamp 1 performs self-excited oscillation for the lighting so as to operate independently of the system. The light transmitted by the film 4 is condensed by the lens 5 and formed into an image on a cell surface of the CCD image sensing device 6. The CCD image sensing device 6 performs photoelectric conversion according to a pulse supplied from the timing generator 7 and outputs an electric signal in accordance with the formed image. The signal outputted from the CCD image sensing device 6 is processed as a video signal by the processing circuit 12 and outputted to an external apparatus (not shown). To obtain a two-dimensional image of the film 4, the film 4 is moved by a motor 8 vertically to the line of pixels of the CCD image sensing device 6.
The level of the output signal of the CCD image sensing device 6 varies according to the depth of the image of the film 4. Since the level of the video signal of the processing circuit 12 varies if the level of the output signal of the CCD image sensing device 6 varies, it is necessary to perform exposure control. In this conventional system, a so-called electronic shutter is provided where the CCD image sensing device 6 is capable of changing the exposure period based on a control signal from an external apparatus and the microcomputer 9 controls the exposure period of the CCD image sensing device 6 to perform exposure control. That is, since the level of the video signal supplied from the processing circuit 12 varies according to the depth of the image of the film 4, the microcomputer 9 detects the level of the output of the processing circuit 12 and controls a timing pulse outputted from the timing generator 7 based on the detection output to control the exposure period of the CCD image sensing device 6. Thus, in the recent image sensing system, exposure control is performed by changing the exposure period (i.e. signal charge accumulation period) of the solid state image sensing device by the electronic shutter without using any mechanical aperture stop or ND filter.
FIGS. 2 and 3 show correspondence relationships among the exposure period of the CCD image sensing device 6, the lighting pulse (current waveform from the inverter power source 2) of the fluorescent lamp 1 and the output signal of the CCD image sensing device 6 when normal exposure is performed (FIG. 2) and when the exposure period is controlled (i.e. when the electronic shutter is used) (FIG. 3). In this case, it is assumed that the film 4 transmits light uniformly, and the shading of the fluorescent lamp 1 serving as a light source, the lens 5, etc. is not considered.
In FIG. 2, the integration values of the lighting pulses of the fluorescent lamp 1 shown by hatched portions a, b and c correspond to the charge accumulation amounts in exposure periods e.sub.1, e.sub.2 and e.sub.3 (e.sub.1 =e.sub.2 =e.sub.3) and correspond to the level of the output signal of the CCD image sensing device 6. In FIG. 3, the integration values of the lighting pulses of the fluorescent lamp 1 shown by hatched portions a', b' and c' correspond to the change accumulation amount in exposure periods E.sub.1, E.sub.2 and E.sub.3 (E.sub.1 =E.sub.2 =E.sub.3) and correspond to the level of the signal of the CCD image sensing device 6 outputted at that time. The periods e.sub.1, e.sub.2 and e.sub.3 and the periods E.sub.1, E.sub.2 and E.sub.3 are also electronic shutter periods.
Pulses P.sub.1, P.sub.2, P.sub.3, . . . in FIG. 3 are timing pulses for the electronic shutter of the CCD image sensing device 6. The electronic shutter is activated in the periods from the fall of the pulses P.sub.1, P.sub.3, P.sub.5, . . . , and P.sub.2n+1 to the rise of the next pulses P.sub.2, P.sub.4, P.sub.6, . . . , and P.sub.2n (exposure period), and the electronic shutter is deactivated in other periods (non-exposure period). The pulses P.sub.1, P.sub.3, P.sub.5, . . . , and P.sub.2n+1 are also used as shift gate pulses (pulses for supplying the charges accumulated by a charge accumulator of the CCD image sensing device 6 to a transferring unit).
When a number of lighting pulses are present in one exposure period as shown in FIG. 2, even if there is a difference in phases of the lighting pulses among exposure periods e.sub.1, e.sub.2 and e.sub.3, the difference in integration values of the lighting pulses among the exposure periods is substantially zero (a=b=c), so that the level of the output signal of the CCD image sensing device 6 is substantially constant.
However, when the exposure period is reduced by performing the exposure control by the electronic shutter as described above, the number of lighting pulses in one exposure periods decreases. In this case, if there is a difference between the phase of the lighting pulse and the phase of exposure, the integration values of the lighting pulses differ among the exposure periods E.sub.1, E.sub.2 and E.sub.3 (a'.noteq.b'.noteq.c') as shown in FIG. 3, so that the level of the output signal of the CCD image sensing device 6 varies. The variation in level of the output signal causes non-uniformity or longitudinal stripes in the image. Such a problem may be solved by further increasing the lighting frequency of the fluorescent lamp 1 as described above. However, the increase in lighting frequency has its limit in view of the cost.